1. Field of the Invention
The present invention relates to a multi-wavelength light source for use in, for example, an optical wavelength division multiplexing communications system and generating a plurality of optical carriers of different wavelengths.
2 . Discussion of the Background
In general, conventional optical wavelength division multiplexing communications systems use a plurality of optical carriers of different wavelengths. Each optical carrier is generated by a separate laser diode. Recently, it was demonstrated that the use of a supercontinuum (SC) and a wavelength separator allows a plurality of optical carriers of different wavelengths to be generated (Electronics Letters, vol. 31, pp. 1064-1066, 1995). The arrangement shown in the paper is shown in FIG. 1.
In FIG. 1, an optical pulse source 11 uses a mode-locked fiber ring laser. This produces a train of optical pulses having a pulse duration of 3.5 ps and a repetition rate of 6.3 GHz.
The optical pulse train is amplified by an erbium doped fiber amplifier (EFDA) 13 so that its peak power reaches about one watt and then introduced into a zero dispersion fiber (SC fiber) 14 that is three kilometers long.
At this point, the spectrum of the optical output of the SC fiber 14 is broadened to as wide as 200 nm by its nonlinearity. This will be described with reference to FIGS. 2A through 2D. FIGS. 2A and 2B respectively show the spectra of input and output pulses of the SC fiber in terms of time domain, while FIGS. 2C and 2D respectively show the spectra of input and output pulses of the SC fiber in terms of wavelength domain.
As shown in FIGS. 2A through 2D, when extremely short optical pulses travel through a nonlinear medium (for example, a dispersion shift fiber), self phase modulation (SPM) occurs, resulting in the generation of a supercontinuum. In this case, the use of pulses having a high peak power causes SPM to occur strongly, allowing the spectrum to be broadened increasingly (refer to "Nonlinear Fiber Optics" by Agrawal).
The use of an optical pulse train having a small duty cycle and a pulse width of the order of a few picoseconds allows an optical amplifier (for example, EDFA) to achieve a peak power of the order of some watts. Further, by passing the optical pulse train through a dispersion shifted fiber, the spectrum width can be broadened to some tens of nanometers. Thus, by extracting a portion of the SC spectrum by means of a wavelength demultiplexer or filter, the extracted portion can be utilized as carriers.
The SC spectrum is composed of components spaced by a frequency that is the same as the pulse repetition frequency. Thus, by extracting a portion of the entire spectrum by means of a wavelength demultiplexer, a plurality of components can be extracted. In terms of time domain, the extracted signal is a pulse train as shown in FIGS. 2A and 2B. Of course, the use of a very narrow filter will make it possible to extract a single component. The wider bandwidth the filter has, the narrower the pulse width will be.
The optical pulse train having its spectrum broadened by the SC fiber 14 in that manner is input into an array waveguide WDM demultiplexer 15. The demultiplexer 15 extracts multiple optical pulse trains .lambda.1 to .lambda.n having different wavelengths from the SC spectrum of the optical pulse train entered. These pulse trains .lambda.1 to .lambda.n are each utilized as an optical carrier.
In the above paper, it is shown that 16 pulse trains spaced by one nanometer in wavelength were generated for use as optical carriers and could be transmitted modulated with data.
However, the conventional multi-wavelength light source arranged as described above has a problem that it is affected by the polarization dependence of an optical modulator. That is, a conventional optical modulator can modulate only optical in a certain plane of polarization. It is therefore required to match the plane of polarization of input light to that plane of polarization. When the above-described multi-wavelength light source is used, therefore, it is required that the mode-locked fiber laser, the EDFA, the dispersion shift fiber, the WDM multiplexer/demultiplexer, the optical modulator, and optical fibers that connect these components be of the polarization-maintaining type.
In the use of polarization maintaining fibers, problems arise in that they not only cost tens of times more than usual fibers but also are difficult and costly to connect. In terms of performance as well, a problem arises in that the polarization extinction ratio goes low.
When the fibers extend one kilometer or more, their performance (polarization extinction ratio) will reduce. Even if polarization maintaining fibers are used, therefore, the plane of polarization will not be held after all, causing variations in peak level in the optical modulator.
With the multi-wavelength light source, the optical fibers causes the nonlinear effect, requiring very high-power light. In order to achieve a high average power, it is required to use an EDFA. It is pump power which can be coupled to an optical fiber that limits the average output power of the EDFA.
By amplifying an optical pulse train having a narrow pulse width and a low repetition frequency in place of continuous light by means of EDFA, an optical pulse train of significantly high peak power can be generated. Since at least one pulse is required to send each bit of data, the maximum allowable bit rate is equal to the pulse repetition frequency. In the use of an optical pulse train that is low in repetition frequency as an optical carrier, therefore, the maximum allowable bit rate will also be limited.